This invention relates to chromate-treated zinc-plated steel strips having high corrosion resistance without coating, good coating adherence, and firm adhesive bond to vinyl chloride and similar resins, as well as a method for making the same.
Most of currently available zinc-plated steel strips are zinc electroplated steel strips and zinc hot dipped or galvanized steel strips. Since they are not necessarily sufficient in corrosion resistance, various zinc alloy plated steel strips including Zn-Ni, Zn-Fe, and Zn-Al alloy plated ones have been developed and marketed. These advanced products may be used as such, but are often used after a chromate treatment which serves for white rust prevention and as a primary treatment for subsequent coating.
Most currently used chromate treatments are reactive chromate treatments which are applied to those products which require a white rust generating time of 24 to 100 hours in the standard salt spray test. In the reactive chromate treatments, the quantity and nature of the resulting chromate film are largely affected by the reactivity of the underlying metal. More particularly, because of their relatively high reactivity, zinc-plated steel strips can be coated with a chromate film only by dipping the strips in conventional chromate solutions having a relatively low etching power. Since zinc alloy-plated steel strips, however, are low reactive, a chromate film can not fully grow thereon in the conventional chromate solutions. Although corrosion resistance is improved by increasing the quantity of a chromate film deposited, an excessively built-up chromate film turns to be yellow due to hexavalent chromium and thus exhibits an undesirable appearance. When such thickly chromated strips are coated with paint, the adherence between the chromate film and the paint is poor.
As a high speed plating line becomes widespread, post-treatment procedures also want speeding up. In order for the reactive chromate treatment to produce a competent quantity of a uniform chromate film, continuous dipping or spraying for a certain period of tim, typically 4 to 10 seconds is necessary. A common approach for accommodating with the high speed line is to increase the number of tanks to extend the reaction time.
Another class of chromate treatment including coating and electrolytic chromate treatments becomes recently available because these treatments are little affected by the reactivity of steel strips and take a short time to completion. The coating chromate treatment is applied to those products which require a corrosion resisting time of 200 hours or more in the standard salt spray test. The electrolytic chromate treatment results in more improved adherence to a coating as compared with the reactive and coating chromate treatments because the resulting chromate film consists essentially of trivalent chromium.
The coating chromate treatment is generally practiced by a method of adding colloidal silica as a film forming agent as disclosed in Japanese Patent Publication No. 42-14050. Another method for conducting the coating chromate treatment involves applying a chromate solution containing an organic polymer by roll coating or dipping and roll squeezing, followed by drying with or without water rinsing. The coating chromate treatment, however, has the disadvantages that it is difficult to control the quantity of a chromate film deposited and that a high speed treatment frequently invites inconsistencies because the chromate film tends to be nonuniform in a transverse direction to the feed direction. It is needed to develop a technique enabling uniform film formation. Another disadvantage is that the resulting chromate film has poor adherence to a coating because the film is thick and retains hexavalent chromium unchanged throughout the film. Also, the chromate film provides a poor adhesive bond to vinyl chloride and similar resins.
The electrolytic chromate treatment is applied by subjecting a steel strip to cathodic electrolysis whereby hexavalent chromium is electrically reduced to trivalent chromium to form a hydrated oxide film at the strip surface. The electrolytic chromate treatment can not only readily accommodate with speeding-up because the quantity of a chromate film can be controlled by a quantity of electricity, but also be applied to various types of steel strips because hexavalent chromium ions in the chromate solution are reduced electrically rather than by redox reaction. The chromate film resulting from the electrolytic chromate treatment consists essentially of trivalent chromium and has higher coating adherence as compared with the reactive and coating chromate treatments, but is less corrosion resistant as compared with the reactive chromate treatment.
One prior art method for carrying out an electrolytic chromate treatment is disclosed in Japanese Patent Publication No. 47-44417 which is incorporated herein by reference. This method is successful in forming a good, but thin chromate film only at a relatively low current density. The chromate layer cannot be further grown even by increasing electricity quantity. Differently stated, the method fails to form a thick chromate film on a zinc alloy plated steel strip. As previously indicated, in general, the electrolytic chromate film is less corrosion resistant as compared with the reactive and coating type chromate films having the same amount of chromium deposited. This is probably because the electrolytic chromate film tends to be porous due to evolution of hydrogen gas during film formation and because the chromate film composed mainly of trivalent chromium contains an insufficient amount of hexavalent chromium to seal such pores or defects, that is, lacks a self-healing ability.
Another method for carrying out an electrolytic chromate treatment is disclosed in Japanese Patent Application Kokai No. 60-110896 which is incorporated herein by reference. A chromate film is formed in a bath containing hexavalent chromium (Cr.sup.6+)+cationic colloidal silica+H.sub.2 SO.sub.4 +optional NaOH for pH adjustment. Due to the inclusion of sulfate residues in the bath, metallic Cr tends to deposit in a chromate film particularly at a high current density and thus, the chromate film often becomes black colored. The cationic colloidal silica and sulfate residues serve as film forming agents while processing inconsistencies often occur. An observation of chromate films under a scanning electron microscope has indicated that chromate films resulting from a bath containing a fluoride additive are more uniform and dense than those from a bath containing sulfuric acid.